In an air-ingesting turbomachine (e.g., a gas turbine), air enters a compressor and is progressively pressurized as it is routed towards a combustor. The compressed air is premixed with a fuel and ignited within a combustion chamber defined within the combustor, thus producing high temperature combustion gases. The combustion gases are then routed from the combustion chamber via a liner and/or a transition piece into a turbine section of the turbomachine where the combustion gases flow across alternating rows of stationary vanes and rotor blades which are secured to a rotor shaft. As the combustion gases flow across the rotor blades, kinetic and/or thermal energy are transferred to the rotor blades, thus causing the rotor shaft to rotate.
To increase turbine efficiency, modern combustors are operated at high temperatures which generate high thermal stresses on various mechanical components disposed within the combustor. As a result, at least a portion of the compressed air supplied to the combustor is used to cool these components. For example, particular combustors include a generally annular cap assembly that at least partially surrounds one or more fuel nozzles within the combustor. Certain cap assembly designs include a cap plate that is disposed at a downstream end of the cap assembly. The fuel nozzles extend at least partially through the cap plate which is typically disposed substantially adjacent to the combustion chamber. As a result, the cap plate is generally exposed to extremely high temperatures.
One way to cool the cap plate is to route a portion of the compressed air into the cap assembly and onto an upstream side of the cap plate. The compressed air is then routed through multiple cooling holes which extend through the cap plate. This method is known in the industry as effusion cooling. However, the compressed air flowing through the multiple cooling holes enters the combustion chamber generally unmixed with the fuel. As a result, emissions such as oxides of nitrogen (NOx) may be exacerbated and turbine efficiency may be decreased. Therefore, an improved system for cooling the cap plate which recirculates the compressed air used to cool the cap plate so that it may be premixed with fuel prior to combustion would be useful.